At the tender young age of 16, she is informally seen as the "head" maid of the house. She is essentially the "perfect" maid—attentive, polite, demure, intelligent, and beautiful. Her devotion is alluded to border on obsessiveness and becomes a major plot point later. (In the original anime series, she had purple hair rather than blue).

She is eventually revealed to have been the product of Project: Blue Silent Bell, a genetic engineering project aimed at creating the ideal maid for Tarou. Her true feelings and emotions are buried underneath layers of emotional programming, and at the end of the series Tarou commits himself to bringing them out. (At any rate, her devotion to and affection for Tarou appear to be sincere, despite the programming.)
(Source: liquisearch)